The present invention refers to a hydraulically releasable coupling, in particular for use together with equipment which is lowered into an oil or gas well.
When working in an oil or gas well, there is a need for introducing different tools and other items into the well. In wells that deviate strongly from the vertical, the tool is often attached to the end of a coiled tube, which in addition to guiding the tool, also enables circulation of the fluid in the well.
It may happen that a tool gets stuck in the well, and special equipment has to be introduced to extract the tool from the well. Before such equipment can be introduced into the well, the coiled tube must be disconnected from the stuck tool and withdrawn from the well. To enable such disconnection of the coiled tube, it is customary to fit a releasable coupling between the coiled tube and tool. Couplings of this kind comprise two sleeve-shaped main parts releasably connected, and secured in coupled position by a releasable lock. A through fluid channel allows fluid to flow from the coiled tube through the coupling, and on to the tool.
The simplest couplings are held together by shear pins which are arranged to break whenever they are subjected to a predetermined force. Detachment from a stuck tool is done by pulling on the coiled tube with sufficient force, so as to make the shear pins break. In deep wells, where there may be a considerable friction between the coiled tube and the wall of the well, it has proved difficult to transmit sufficient power to break the shear pins, and therefore they must be dimensioned to break by a relatively small force. This easily results in the shear pins breaking unintentionally, for example by vibrations and shock caused by the tool working in the well. To alleviate this problem, it is known to lock the two main parts of the coupling together by means of a locking device, which is kept in locking position by a displaceable locking sleeve, and in which the locking sleeve is kept in position by shear pins. In such known arrangements the shear pins are not subjected to shear forces when the tool is in ordinary use. Disconnecting is done by dropping a sealing body, typically a ball, through the coiled tube and down into the coupling, where the ball lands on a seat, assigned to the locking sleeve, and blocks the through fluid channel. Increasing the fluid pressure in the coiled tube, gives rise to a hydraulic force against the sealing body, and thus against the sleeve. If the fluid pressure is sufficiently increased, the force will be great enough to break the shear pins and displace the locking sleeve, so that the coupling is released. Such hydraulically releasable couplings have, because of their functional reliability, become widely used.
Some of the hydraulic tools require hydraulic control signals in addition to hydraulic power, and it is common to use a coiled tube, prefitted with two internal thin tubes, for the transmission of such hydraulic control signals. In addition the coiled tube often carries an electric cable for the transmission of electric signals to or from the tool. In such cases there is no room for dropping a sealing body through the coiled tube, and known couplings which are released by means of a sealing body, can, therefore, not be used. Thus, couplings released through pull is the only possibility left, as mentioned above.